Technology Computer Aided Design (TCAD) and the corresponding semiconductor device modeling and simulation, such as transistor modeling and simulation, are vital in developing advanced CMOS (Complementary Metal-Oxide-Semiconductor) technology and in achieving reliable performance from circuit designs using semiconductor devices. Moreover, TCAD semiconductor device modeling and simulation can significantly reduce the technology development time and cost and can increase the efficiency of the corresponding circuit design process.
For many years drift-diffusion (DD) has been the workhorse of CMOS transistor TCAD. However the usefulness of the traditional drift-diffusion TCAD in advanced CMOS has become questionable. The development of comprehensive mobility models, which are at the heart of any drift-diffusion TCAD simulators, has become a very expensive and time consuming task. Even in the case of silicon-channel FinFETs, for example, this includes modeling different orientations of the substrate and the fin, different and sometimes complex interface orientations and different strain conditions. Having all necessary measurements to inform the mobility model development and parameter extraction becomes difficult, computationally expensive and slow. Having them before the corresponding FinFET technology is developed is practically impossible. The problem is exacerbated for transistors with new channel materials such as SiGe, Ge or III-Vs, rendering the use of drift-diffusion for screening of new technology options practically impossible. If 50 years of silicon CMOS were not enough to develop all-encompassing silicon mobility models, the prospects for developing predictive simulations for transistors with new channel materials using conventional approaches are poor. Furthermore, even if accurate low-field mobility models in the above cases were to be developed using conventional approaches, their value for predictive drift-diffusion simulation of transistor performance at high drain bias conditions is very limited. Transistor performance these days is determined by non-equilibrium, quasi-ballistic transport, which is beyond the reach of the drift-diffusion simulators. Only ensemble Monte Carlo (EMC) simulations can properly capture the high field, quasi-ballistic, transport in contemporary and future CMOS transistors and can predict their performance.
TCAD practices are therefore shifting towards the use of EMC simulations to predict the transistor performance of present and future CMOS transistors. EMC simulations directly link the mobility and non-equilibrium, quasi-ballistic, transport properties to basic material properties such as the band structure and include the relevant scattering processes that determine carrier dynamics and therefore performance. However EMC simulations are slow, noisy and expensive.
Drift-diffusion simulations calibrated to EMC simulations can be used for technology and transistor optimization, for variability and reliability simulation, for generation of target characteristics and for compact model extraction providing the basis for Design-Technology Co-Optimization (DTCO). However, it is not practical to perform the required comprehensive EMC simulations for any new transistor geometry, new substrate or channel orientation, new strain conditions, new channel materials, etc.